The invention is based on a fuel injection device as generally described hereinafter. Many variant embodiments of a fuel injection device of this kind are already known, the pump piston of which, moved with a constant stroke within a pump cylinder, drives a free piston operating within the same pump cylinder via a fluid column enclosed within a compensation chamber and acting as a hydraulic push rod. The quantity of fuel to be injected is pre-stored below the free piston by a metering device, and the fluid column balances out the corresponding pre-storage stroke so that either a constant supply onset determined by a fixed control opening in the pump cylinder is controlled, as is the case with a pump/nozzle as described in German Pat. No. 820,823, for example, or else a variable supply onset is controlled which is likewise independent of the injection quantity but can be influenced by separate control means. A variable supply onset of this kind is determined, in a pump/nozzle as described by German Offenlegungsschrift No. 25 58 699, by an oblique control edge on the end of the pump piston. In a pump/nozzle as shown in FIGS. 14 and 15 of U.S. Pat. No. 4,134,549, both the fuel injection quantity pre-stored below the free piston and the volume of the fluid column located in the compensation chamber are acted upon by two different fuel pressure sources, which have different supply pressures; the inflow pressure and the opening duration of inflow throttles opened at bottom dead center of the pistons, respectively, determine the quantity of fuel stored in the chambers. In all the known instances, a control edge on the free piston opens up an outflow conduit at the end of the supply stroke; as a result, the supply movement of the free piston is terminated, and the associated injection nozzle terminates the injection of fuel.
For the sake of improved power and improved exhaust emissions, a rapid needle closure is required in modern Diesel engines, and in the pump/nozzle according to German Offenlegungsschrift No. 25 58 699, this is attained by means of a pressure relief controlled by the free piston of the pressure line leading to the nozzle. A portion of the fuel located in the pump work chamber flows out into a return flow line, so that this outflow quantity must be included in the next metering of an injection quantity. Since the outflow quantities are dependent on both the rpm and the fuel injection quantity at a given time, this outflow quantity may influence the quantity metering in a disadvantageous manner if heavy demands are made on precision of metering. In the other known pump/nozzles without pressure relief of the pump work chamber, the disadvantage of a retarded end of injection occurs, causing after-injections and a worsening of the exhaust gas composition.
In order to attain optimal power over the entire rpm range and in order to adhere to the ever-more stringent requirements for preserving air quality, a fuel injection device is required which retains the known advantages of a control of the supply onset which is independent of the prestored fuel injection quantity and additionally enables the pressure relief of the pump work chamber at end of injection, without negatively affecting the precision of fuel quantity metering.